DC--DC converter techniques which are known in the art include pulse width modulation (PWM) DC--DC converters and resonant series and parallel converters. Such methods are known to be susceptible to switching losses, particularly in the former case, and conduction losses.
In a conventional PWM DC--DC converter using switching transistors, the switches have an inherent capacitance which results in a residual voltage across the switch when the switches are alternately open and closed. Accordingly, it is desirable to delay switching to allow discharge of the capacitance by placing an inductor in series with the load resistance. This will resist the change in the current flow due to the residual field in the inductor. With no inductance, the power factor is close to unity. If an inductor is added in series with the load the power factor lags and soft switching is provided.
Ideally the current and voltage in a circuit element will be in phase. However, if there is a slight phase difference there will be instants where the current is positive and the voltage negative (or vice versa). In this situation the instantaneous power will be negative and the power transmitted will be effectively reduced. Thus the power transmission is less for the same time averaged values of voltage and current.
To obtain the same or a desired level of power transmission, the voltage must be increased but this increases the current correspondingly which leads to increased I.sup.2 R losses (or conduction losses). The traditional method of reducing output voltage in a PWM converter is to reduce the duty cycle. But as the duty cycle is reduced, the peak current is increased if the same average current is to be maintained. However, as the peak current is increased the I.sup.2 R losses become disproportionately high.
In presently known resonant circuits, it is possible to reduce the voltage by putting a reactance in series with the load. However, this may significantly impair the power factor. The present invention avoids the need for such a reactance and further provides an inductance to effect a lagging power factor (or soft switching). The present invention further provides for a converter where the turns ratio may be varied (and therefore the conversion ratio adjusted) while retaining the use of all of the primary windings.
It is an object of the present invention to overcome or at least mitigate some of the disadvantages inherent in the prior art or to at least provide the public with a useful choice.